Off-axis catadioptric projection optical system for lithography

ABSTRACT

The present invention is directed to off-axis catadioptric projection optical systems for use in lithography tools for processing modulated light used to form an image on a substrate, such as a semiconductor wafer or flat panel display. In one embodiment the optical system includes an off-axis mirror segment, a fold mirror, a relay, an aperture stop and a refractive lens group. Modulated light is transmitted through the system to form an image on a substrate. In a second embodiment the projection system includes an off-axis mirror segment, an aperture stop and a refractive lens group. In a third embodiment the projection system includes an off-axis mirror segment, a negative refractive lens group, a concave mirror, a relay, an aperture stop, and a refractive lens group. A method to produce a device using a lithographic apparatus including a projection system with an off-axis mirror segment as the first element in a projection optics system is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lithography, and more particularly, toprojection optics used in lithography.

2. Background of Invention

Lithography systems are used to print features in a variety ofmanufacturing applications. Photolithography systems use a mask orreticle to expose features onto an object. In semiconductormanufacturing, for example, a reticle is exposed by an exposure beam. Anoptical system then projects a reduced image of the reticle onto asilicon wafer. In this way, circuit features can be printed on asemiconductor substrate.

Maskless lithography systems have been developed that do not require useof a mask or reticle. Current maskless lithography systems project apattern to be printed onto a moving object. For example, a pattern ofcircuit features can be projected onto a moving wafer or flat paneldisplay. In one example, a silicon wafer can be coated with aphotoresist. The pattern is projected on the wafer using one or morepattern generating devices, suchas spatial light modulator (SLM) arrays.This SLM array is a programmable array of elements that modulates thelight projected onto the object. Types of SLMs can include, for example,digital micromirror devices (DMD), transmissive liquid crystal lightvalves (LCLV), and grating light valves (GLV).

Projection optical systems are used to transmit light generated by apattern generator, such as an SLM or a static light source used inlithography systems that use masks, to a wafer to create an image on thewafer. Existing projection systems have significant limitations. Amongthese limitations are that existing systems often include a very largenumber of optical components and are often complicated. For example, inone case the projection optical system includes three relays. The firstrelay is an on-axis module, while the other two are off-axis modules.Each module must be corrected independently. Aberrations of on-axis andoff-axis modules are not compensable. Furthermore, the alignment processis complicated and overall dimensions are large. Other limitations arefound in systems that limit light transmission to only about 25% of thelight provided by the pattern generating device.

What is needed is a projection system that provides high performancewith reduced complexity.

SUMMARY OF THE INVENTION

The present invention is directed to off-axis catadioptric projectionsystems for use in lithography tools, such as a lithographic projectionapparatus, for processing modulated light used to form an image on asubstrate, such as a semiconductor wafer. A catadioptric system is onein which both reflective and refractive optical elements are used. Anumber of embodiments are disclosed. In one embodiment the projectionsystem includes a pattern generator, an off-axis mirror segment, arelay, an aperture stop and a refractive lens group. Modulated light istransmitted through the system to form an image on a substrate. Aprimary advantage of this embodiment is that the off-axis mirror segmentcan be used to reduce field curvature.

In a second embodiment the projection system includes a patterngenerator, an off-axis mirror segment, a fold mirror, a relay, anaperture stop and a refractive lens group. Like the first embodiment,this embodiment reduces field curvature of the modulated light, andprovides greater design flexibility compared to the first embodiment.

In a third embodiment the projection system includes a patterngenerator, an off-axis mirror segment, a negative refractive lens group,a concave mirror, a relay, an aperture stop and a refractive lens group.This embodiment also reduces field curvature of the modulated light. Theuse of the concave mirror allows further reduction of field curvature.Additionally, the use of the negative refractive lens group reduceschromatic aberrations, and allows the relay and refractive lens groupdesigns to be simplified.

A method to produce a device using a lithographic apparatus including aprojection system with an off-axis mirror segment as the first elementin a projection optics system is also provided.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1 is a diagram of a lithographic projection apparatus, according toan embodiment of the invention.

FIG. 2 is a diagram of a projection system with an off-axis mirrorsegment for use in a lithography tool, according to an embodiment of thepresent invention.

FIG. 3 is a diagram of a projection system with an off-axis mirrorsegment for use in a lithography tool, according to an embodiment of thepresent invention.

FIG. 4 is a diagram of a projection system with an off-axis mirrorsegment for use in a lithography tool, according to an embodiment of thepresent invention.

FIG. 5 is a flowchart of a method to produce a device using alithographic apparatus including a projection system with an off axismirror segment, according to an embodiment of the invention

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the invention would be ofsignificant utility.

The term “pattern generator” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, so that adesired pattern can be created in a target portion of the substrate. Theterms “light valve” and “Spatial Light Modulator” (SLM) can also be usedin this context. Examples of such patterning devices are discussedbelow.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate. An array of diffractive optical micro electrical mechanicalsystem (MEMS) devices can also be used in a corresponding manner. Eachdiffractive optical MEMS device can include a plurality of reflectiveribbons that can be deformed relative to one another to form a gratingthat reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror arrayemploying a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means.

In both of the situations described here above, the pattern generatorcan comprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO98/38597 and WO 98/33096, which are incorporated herein by reference intheir entireties.

A programmable LCD array can also be used. An example of such aconstruction is given in U.S. Pat. No. 5,229,872, which is incorporatedherein by reference in its entirety.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe pattern generator may differ substantially from the patterneventually transferred to a layer of or on the substrate. Similarly, thepattern eventually generated on the substrate may not correspond to thepattern formed at any one instant on the pattern generator. This may bethe case in an arrangement in which the eventual pattern formed on eachpart of the substrate is built up over a given period of time or a givennumber of exposures during which the pattern on the pattern generatorand/or the relative position of the substrate changes.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus and projection systemsdescribed herein may have other applications, such as, for example, themanufacture of DNA chips, MEMS, MOEMS, integrated optical systems,guidance and detection patterns for magnetic domain memories, flat paneldisplays, thin film magnetic heads, etc. The skilled artisan willappreciate that, in the context of such alternative applications, anyuse of the terms “wafer” or “die” herein may be considered as synonymouswith the more general terms “substrate” or “target portion”,respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically appliesa layer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Where applicable, the disclosure hereinmay be applied to such and other substrate processing tools. Further,the substrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” and “light ray” used herein encompassall types of electromagnetic radiation, including ultraviolet (UV)radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation.

The lithographic apparatus may be of a type having two (e.g., dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.,water), so as to fill a space between the final element of theprojection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

FIG. 1 is a diagram of lithographic projection apparatus 100, accordingto an embodiment of the invention. Apparatus 100 includes at least aradiation system 102, pattern generator 104, an object table 106 (e.g.,a substrate table), and an projection system 108.

Radiation system 102 can be used for supplying a projection beam 110 ofradiation (e.g., UV radiation), which in this particular case alsocomprises a radiation source 112.

An pattern generator 104 (e.g., spatial light modulator) can be used forapplying a pattern to projection beam 110. In general, the position ofpattern generator 104 can be fixed relative to projection system 108.However, in an alternative arrangement, pattern generator 104 may beconnected to a positioning device (not shown) for accurately positioningit with respect to projection system 108. As here depicted, patterngenerator 104 is of a reflective type (e.g., have a reflective array ofindividually controllable elements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device (not shown) for accurately positioning substrate114 with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF2 lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 may project animage of pattern generator 104 onto substrate 114. Alternatively,projection system 108 may project images of secondary sources for whichthe elements of pattern generator 104 act as shutters. Projection system108 may also comprise a micro lens array (MLA) to form the secondarysources and to project microspots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander 126, for example. Illuminator 124 may comprise anadjusting device 128 for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in beam 122. In addition, illuminator 124 willgenerally include various other components, such as an integrator 130and a condenser 132. In this way, projection beam 110 impinging onpattern generator 104 has a desired uniformity and intensitydistribution in its cross section.

It should be noted, with regard to FIG. 1, that source 112 may be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 may also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts pattern generator 104 after beingdirected using beam splitter 118. Having been reflected by patterngenerator 104, beam 110 passes through projection system 108, whichfocuses beam 110 onto a target portion 120 of the substrate 114.

With the aid of positioning device (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 106 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the patterngenerator 104 can be used to accurately correct the position of patterngenerator 104 with respect to the path of beam 110, e.g., during a scan.In general, movement of object table 106 is realized with the aid of along-stroke module (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem may also be used to position pattern generator 104. It will beappreciated that projection beam 110 may alternatively/additionally bemoveable, while object table 106 and/or pattern generator 104 may have afixed position to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106may be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 may be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 may be used to project a patterned projection beam 110 for use inresistless lithography.

The depicted apparatus 100 can be used in four preferred modes:

1. Step mode: the entire pattern from pattern generator 104 is projectedin one go (i.e., a single “flash”) onto a target portion 120. Substratetable 106 is then moved in the x and/or y directions to a differentposition for a different target portion 120 to be irradiated bypatterned projection beam 110.

2. Scan mode: essentially the same as step mode, except that a giventarget portion 120 is not exposed in a single “flash.” Instead, patterngenerator 104 is movable in a given direction (the so-called “scandirection”, e.g., the y direction) with a speed v, so that patternedprojection beam 110 is caused to scan over pattern generator 104.Concurrently, substrate table 106 is simultaneously moved in the same oropposite direction at a speed V=Mv, in which M is the magnification ofprojection system 108. In this manner, a relatively large target portion120 can be exposed, without having to compromise on resolution.

3. Pulse mode: the array of individually controllable elements 104 iskept essentially stationary and the entire pattern is projected onto atarget portion 120 of substrate 114 using pulsed radiation system 102.Substrate table 106 is moved with an essentially constant speed suchthat patterned projection beam 110 is caused to scan a line acrosssubstrate 106. The pattern on pattern generator 104 is updated asrequired between pulses of radiation system 102 and the pulses are timedsuch that successive target portions 120 are exposed at the requiredlocations on substrate 114. Consequently, patterned projection beam 110can scan across substrate 114 to expose the complete pattern for a stripof substrate 114. The process is repeated until complete substrate 114has been exposed line by line.

4. Continuous scan mode: essentially the same as pulse mode except thata substantially constant radiation system 102 is used and the pattern onpattern generator 104 is updated as patterned projection beam 110 scansacross substrate 114 and exposes it.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 is a diagram of projection system 200 with an off-axis mirrorsegment for use in a lithography tool, such as lithographic projectionapparatus 200, according to an embodiment of the invention. Projectionsystem 200 includes pattern generator 210, off-axis mirror segment 220,aperture stop 230, and refractive lens group 240.

Pattern generator 210 modulates light for use in forming an image on awafer or other substrate. Pattern generator 210 can be a spatial lightmodulator (SLM), but is not limited to an SLM. Light rays, such as rays255, 260, 265 and 270 are transmitted from pattern generator 210 tooff-axis mirror segment 220. Rays 255 and 265 are marginal rays, whilerays 260 and 270 are chief rays. Line 205 represents the optical axis ofoptical system 200.

Off-axis mirror segment 220 receives light rays and reflects those lightrays. Off-axis mirror segment 220 can be concave, which is conducive forfield curvature correction. Light rays 255, 260, 265 and 270 arereflected from off-axis mirror segment 220 through aperture stop 230.Upon passing through aperture stop 230, light rays 255, 260, 265 and 270impinge upon lens 240. Upon receiving light rays, refractive lens group240 provides the main optical power of optical system 200 and focusesthe light rays onto substrate 250 to form an image on substrate 250.Refractive lens group 240 can be a single refractive element or a set ofrefractive elements. Aperture stop 230 should be placed at front focalpoint of refractive lens group 240. Substrate 250 is positioned at theimage plane of the whole optical system including off-axis mirrorsegment 220 and refractive lens group 240.

FIG. 3 is a diagram of projection system 300 with an off-axis mirrorsegment for use in a lithography tool, such as lithographic projectionapparatus 100, according to an embodiment of the present invention.Projection system 300 provides similar functionality to projectionsystem 200, but allows for greater flexibility in the placement of awafer or substrate to be imaged.

Projection system 300 includes pattern generator 305, off-axis mirrorsegment 310, fold mirror 315, relay 320, aperture stop 325, andrefractive lens group 330. Pattern generator 305 modulates light for usein forming an image on a wafer or other substrate. Pattern generator 305can be a spatial light modulator (SLM), but is not limited to an SLM.Light rays, such as rays 355, 360, 365 and 370 are transmitted frompattern generator 305 to off-axis mirror segment 310. Rays 355 and 365are marginal rays, while rays 360 and 370 are chief rays. Line 345represents the optical axis of optical system 300.

Off-axis mirror segment 310 receives light rays and reflects those lightrays. Off-axis mirror segment 310 can be concave, which is conducive forfield curvature correction. Light rays 355, 360, 365 and 370 arereflected from off-axis mirror segment 310 to fold mirror 315. Foldmirror 315 reflects the light rays toward relay 320. The use of foldmirror 315 enables greater flexibility in the placement of wafer 340,than was the case in projection system 200. An intermediate image 375 isformed that occurs prior to relay 320.

Relay 320 includes a set of refractive lens and mirrors as will be knownby individuals skilled in the relevant arts. Relay 320 provides opticalpower for projection system 300 and passes the light rays throughaperture stop 325. Upon passing through aperture stop 325, light rays355, 360, 365 and 370 impinge upon refractive lens group 330. Uponreceiving light rays, refractive lens group 330 provides additionaloptical power for projection system 300 and focuses the light rays ontosubstrate 340 to form an image on substrate 340. Refractive lens group330 can be a single refractive element or a set of refractive elements.Aperture stop 325 should be placed at front focal point of refractivelens group 330.

FIG. 4 is a diagram of projection system 400 with an off-axis mirrorsegment for use in a lithography tool, such as lithographic projectionapparatus 100, according to an embodiment of the invention. Projectionsystem 400 provides similar functionality to projection system 200, butallows for greater flexibility in the placement of a substrate to beimaged. Projection system 400 also provides for greater field curvaturecorrection and reduction of chromatic aberrations than either projectionsystem 200 or 300.

Projection system 400 includes pattern generator 405, off-axis mirrorsegment 410, negative refractive lens group 415, fold mirror 420, relay425, aperture stop 430, and refractive lens group 435. Pattern generator405 modulates light for use in forming an image on a wafer or othersubstrate. Pattern generator 405 can be a spatial light modulator (SLM),but is not limited to an SLM. Light rays, such as rays 455, 460, 465 and470 are transmitted from pattern generating device 405 to off-axismirror segment 410. Rays 455 and 465 are marginal rays, while rays 460and 470 are chief rays. Line 445 represents the optical axis ofprojection system 400.

Off-axis mirror segment 410 receives light rays and reflects those lightrays. Off-axis mirror segment 410 can be concave, which is conducive forfield curvature correction. Light rays 455, 460, 465 and 470 arereflected from off-axis mirror segment 410 to negative refractive lensgroup 415. Negative refractive lens group 415 can include one or morerefractive lens elements. In an embodiment negative refractive lensgroup 415 includes negative refractive lens elements. Negativerefractive lens group 415 can be used to correct chromatic aberrationsin the modulated light, thereby simplifying other lens groups withinprojection system 400. Upon passing through negative refractive lensgroup 415, the modulated light reflects off of concave mirror 420.Concave mirror 420 provides further field curvature correction. Concavemirror 420 is positioned close to negative refractive lens group 415. Inan embodiment, concave mirror 420 is positioned as close as possible torefractive lens group 415. Concave mirror 420 reflects the modulatedlight back through negative refractive lens group 415. An intermediateimage 475 is formed after the modulated light passes through negativerefractive lens group 415.

The modulated light is then transmitted through relay 425. Relay 425includes a set of refractive lens and mirrors as will be known byindividuals skilled in the relevant arts. Relay 425 provides opticalpower for projection system 400 and passes the light rays throughaperture stop 430. Upon passing through aperture stop 430, light rays455, 460, 465 and 470 impinge upon refractive lens group 435. Uponreceiving light rays, refractive lens group 435 provides additionaloptical power for projection system 400 and focuses the light rays ontosubstrate 440 to form an image on substrate 440. Refractive lens group435 can be a single refractive element or a set of refractive elements.Aperture stop 430 should be placed at the front focal point ofrefractive lens group 435.

FIG. 5 is a flowchart of method 500 to produce a device using alithographic apparatus including a projection system with an off axismirror segment, according to an embodiment of the invention. Method 500begins in step 510. In step 510, a projection beam is emitted. Forexample, radiation system 102 can be used to emit a projection beam. Instep 520, a patterned projection beam is generated based on theprojection beam emitted is step 510. For example, pattern generator 104can be used to apply a pattern to the emitted projection beam. In step530, the patterned projection beam is reflected using an off-axis mirrorsegment, such as for example, off-axis mirror segment 220. In step 540,the reflected patterned projection beam generated in step 530 isfocused. Focusing can occur through the use of a combination ofreflective and refractive elements as will be known by individualsskilled in the relevant arts, based on the teachings herein. In step550, a device is created by imaging the reflected patterned projectionbeam onto a substrate, as will be known by individuals skilled in theart, based on the teachings herein. Example devices can include, but arenot limited to, integrated circuits and flat panel displays. In step560, method 500 ends.

In general, the invention has several advantages, including but notlimited to the following. The use of an off-axis mirror segment as thefirst element receiving light from a pattern generating device, such asa spatial light modulator, for example, enhances the robustness of thesystem. Specifically, inhomogeneities in the off-axis mirror do notaffect system performance. Furthermore, wedge errors that typicallydegrade performance in other systems are not relevant and do not impactsystem performance. As a result, an off-axis mirror segment that islarger in size than typical first elements in a projection opticalsystem can be used, and therefore the total spatial light modulator areacan be increased. Moreover, the use of concave mirrors are conducive forthe correction of field curvature, which in turn can simplify therefractive portions of the optical system.

CONCLUSION

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

1. A projection system for use in a lithography tool for processing modulated light used to form an image on a substrate, comprising: an off-axis mirror segment that receives modulated light; an aperture stop that receives the modulated light from said off axis mirror segment and; and a refractive lens group that focuses the modulated light onto the substrate.
 2. The optical system of claim 1, further comprising a pattern generating device that provides the modulated light for use in forming the image on the substrate.
 3. The optical system of claim 2, wherein said pattern generating device is a spatial light modulator.
 4. The optical system of claim 1, wherein said off-axis mirror segment is concave.
 5. The optical system of claim 1, wherein said off-axis mirror segment provides field curvature correction.
 6. The optical system of claim 1, wherein said substrate is a semiconductor wafer or flat panel display.
 7. An optical system for use in a lithography tool for processing modulated light used to form an image on a substrate, comprising; an off-axis mirror segment that receives modulated light; a fold mirror that receives the modulated light from said off-axis mirror segment; a relay that receives the modulated light from said fold mirror and focusing the modulated light; an aperture stop that receives the modulated light from said relay; and a refractive lens group that focuses the modulated light onto the substrate.
 8. The optical system of claim 7, further comprising a pattern generating device that provides the modulated light for use in forming an image on the substrate.
 9. The optical system of claim 8, wherein said pattern generating device is a spatial light modulator.
 10. The optical system of claim 8, wherein said off-axis mirror segment is concave.
 11. The optical system of claim 8, wherein said off-axis mirror segment provides field curvature correction.
 12. The optical system of claim 8, wherein said substrate is a semiconductor wafer or flat panel display.
 13. An optical system for use in a lithography tool for processing modulated light used to form an image on a substrate, comprising; an off-axis mirror segment that receives modulated light; a concave mirror that receives modulated light from said off-axis mirror segment; a negative refractive lens group positioned between said off-axis mirror segment and said concave mirror; a relay that receives light from said concave mirror that has passed through said refractive lens group and that focuses the modulated light; an aperture stop that receives the modulated light from said relay; and a refractive lens group that focuses the modulated light onto the substrate.
 14. The optical system of claim 13, further comprising a pattern generating device that provides the modulated light for use in forming an image on the substrate.
 15. The optical system of claim 14, wherein said pattern generating device is a spatial light modulator.
 16. The optical system of claim 13, wherein said off-axis mirror segment is concave.
 17. The optical system of claim 13, wherein said off-axis mirror segment provides field curvature correction.
 18. The optical system of claim 13, wherein said negative refractive lens group provides chromatic aberration reduction.
 19. The optical system of claim 13, wherein said substrate is a semiconductor wafer or a flat panel display.
 20. A lithographic apparatus, comprising: an illumination system that supplies a projection beam of radiation; a pattern generator that patterns the projection beam; and a projection system that projects the patterned beam along a predetermined beam path onto a substrate, wherein said projection system comprises an off-axis mirror segment that receives the patterned beam from said pattern generator.
 21. A method to produce a device using a lithographic apparatus comprising a projection system with an off axis mirror segment, comprising the steps of: (a) emitting a projection beam; (b) generating a patterned projection beam based on said radiation beam; (c) reflecting said patterned projection beam with the off-axis mirror segment; (d) focusing the reflected patterned projection beam; and (e) imaging the reflected patterned projection beam onto a substrate to create the device.
 22. The method of claim 21, wherein step (e) comprises imaging the reflected patterned projection beam onto a substrate to create an integrated circuit.
 23. The method of claim 21, wherein step (e) comprises imaging the reflected patterned projection beam onto a substrate to create a flat panel display. 